The remains of the Kladnitsa fortress are located of south-western slopes of the Vitosha mountain, near the village of Kladnitsa. The fortress was built on the hill and was naturally protected because there are very steep slopes around its three sides. Kladnitsa fortress was included in the system of mountain fortresses defending the approaches to town of Serdika (nowadays Sofia) and it had a visual contact with other forts in the region.
The fortress was relatively large (about 10 000 m2) and had 2 levels. The higher part had administrative functions while the lower was for the troops and the weapon workshops. Its entrance was from the southeast. There was an underground tunnel, which led to the opposite hill, and was used in the siege. The walls were strong and double, built of stone and mortar. Today, remnants of them have a height a little higher than half a meter.
In the region have been found numerous artifacts - carved stone map of the castle, edged by arrows, spears and knives, fine yellowish glass, coins, pottery, decorated with geometric and floral ornaments and more. In higher (the administrative) part is revealed foundations of the basilica.
Nowadays the ruins of the castle are overgrown with bushes and access to them is difficult because the path goes through a private property.
Not far away from the fortress ruins is located Kladnitsa monastery.